Fix 13 typos in comments, docs and site copy

[Avicennasis]

Summary

Fixes 13 typos across 10 files - code comments, plus one user-facing sentence on the site. One change is not a comment; it is called out separately below.

Behaviour change - please review this one carefully

• togetherjs/forms.js: this._editor().destory(); -> this._editor().destroy(); inside the TinyMCE tracker's destroy() method. _editor() returns a TinyMCE editor instance ($(this.element).data("tinyEditor")) and the TinyMCE API method is destroy(), so the current call raises a TypeError. destory occurs exactly once in the whole repository.

Site copy

• site/index.html and site/js/custom.js: a unique TogetherJS url to a friend -> URL. The same sentence is duplicated in both files, and site/docs/index.md uses URL throughout.

Comment typos

• togetherjs/videos.js: uneccesary -> unnecessary
• togetherjs/session.js: succede -> succeed; arives -> arrives; weather -> whether
• togetherjs/walkthrough.js: substititions -> substitutions
• togetherjs/youtubeVideos.js: supress -> suppress; everytime -> every time
• togetherjs/ot.js: preceed -> precede
• togetherjs/jqueryPlugins.js: dimenensions -> dimensions
• togetherjs/ui.js: draggin -> dragging

No files under togetherjs/libs/, togetherjs/tests/doctestjs/ or site/examples/friendlycode/ were touched - those are third-party/vendored copies.